On IOS14, you must init the playback interface. SpriteKit use AVFundation to play sound with SKAudioNode.
In the appDelegate, do :
#import "AppDelegate.h"
@import AVFAudio;
@implementation AppDelegate
(BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
AVAudioSession *audioSession = [AVAudioSession sharedInstance];
[audioSession setCategory:AVAudioSessionCategoryPlayback error:nil];
[audioSession setActive:YES error:nil];
return YES;
}
You could also include the AVAudio.framework in the list of your frameworks.